Process-based modelling of river flow and nitrate loadings in the Ythan catchment, Scotland

The Ythan catchment has been undergoing a steady increase in nitrate concentration over the years with a significant increase after 1990. Nitrogen transformations in soil and its transport through the catchment is a very complex process. Modelling can help in explaining this complexity. In this study, TOPMODEL and SWRRBWQ models are applied to the Ythan catchment. TOPMODEL performance in simulating catchment hydrological responses is assessed to test the ability of the model structure to simulate basic transport processes in the Ythan catchment. The result of TOPMODEL simulation shows the ability of the model to simulate total runoff with an accuracy of 95 % while the efficiency of the model in simulating runoff pattern is between 74 % and 54 %. SWRRBWQ is a process based hydrological and water quality model which allows the impact of changing agricultural management practices to be simulated. In this study the performance of SWRRBWQ in simulating catchment hydrological responses and nitrate loadings is assessed. Modelling result shows a good performance in modelling monthly runoff pattern with high correlation of 0.92 for calibration period (1991), 0.87 for 1990, and 0.74 for 1993; nevertheless the model underpredicts total runoff quantity throughout the three years under scrutiny. SWRRBWQ is found to be useful as a tool to evaluate the sensitivity of soil parameters on nitrate leaching and to study the impact of changing agricultural management patterns on nitrate loadings. The model results show a high tendency for nitrate leaching in soil with high conductivity. Modelling of rate of fertilisation and cropping pattern shows that nitrate loadings increase with increasing nitrate fertiliser use; 13, 20, and 78 kgha-1 for application rates of 78, 100, and 200 kgha-1 respectively The model also simulated the order of increasing nitrate loadings according to crop in the following manner: grass; winter wheat > winter barley > spring barley